The Bitterling, or Bitter Carp (Rhodeus amarus) is one of the smallest of European fresh-water fishes, the male being, when fully grown, about 3in. long, while the female is only 2in., or even less. Its appearance suggests a cross between the Common and the Prussian Carp. The dorsal fin commences at the middle of the back, and ends a little beyond the centre of the anal fin. The pectoral and ventrals are of equal size; the anal fin being nearly as large as the dorsal. The caudal fin is moderately concave. The vent, which is surrounded by a kind of scaly sheath, is midway between the anal and ventral fins.

The male, during spawning times, assumes very beautiful colours, for which it has received the name of "Rainbow-coloured fish". The Gill covers, back, and the sides are of a beautiful violet colour, while on each side of the body, beginning at the centre of the caudal fin, is a bright green stripe reaching nearly to the middle of the body.Behind the gill-covers there is a silvery patch spotted with violet. The anal fin is a bright red edged with black. Altogether, the Bitterling during summer is one of the most beautiful of European fishes, as well as one of the most hardy.

The female, though not nearly so beautiful as the male, is, nevertheless, a handsome little fish. Her back is greenish-brown and her sides silvery. The line running from the centre of the caudal fin towards the middle of the body is of a blackish colour, and is sometimes absent

She has a curious possession in the shape of a long reddish oviduct, which sometimes hangs down from the body as much as 1in., or even more. After the spawning is ended it gradually disappears.

It is not known, I think, whether this tube, which is said to possess its own nerves and blood vessels, is used as an ovipositor, or not.

The size of the Bitterling, and its bitter flavour debar it from being useful as a food for man. As a bait for other fish it rarely has any attraction, except for Eels, and sometimes Perch.

It is widely distributed throughout the centre of Europe. It is not found as a native in Great Britain.

As food it will take, while in captivity, vermicelli, small worms, and ants eggs.

The loveliest and most interesting of European freshwater fishes, very well suited to the domestic aquarium and in its beautiful coloration at spawning time excelled by few exotic species. Outside the breeding season both sexes are similarly coloured. Back grey-green; flanks and belly gleaming silver. Under the dorsal fin begins a shining grey-green stripe which ends at the rot of the tail. Dorsal fin blackish; all other fins delicate reddish or yellowish.

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Care in well-planted tanks with a layer of mulm and without aeration. Temperature up to 22°C. Omnivorous, with a preference for enchytraids and midge larvae. It is important to keep Swan-Mussels (Anodonta spp.) in the tanks as well, as the Bitterling needs these or spawning. Spawns readily in April after a cold wintering. etc.

Sometimes known as the Rainbow Carp, the male is vividly coloured during mating season. At other times, the male is silver with a touch of rose on the fins. The female is usually silver with a yellowish back and lateral blue stripes. They look rather like washed-out Goldfish.

Community tank life seems to agree with this fish. Its eggs are deposited in fresh-water mussels and these mussels must be present in order to breed Bitterlings. This applies to the European natives only, for the American Bitterling is not known to breed with the use of mussels. Temperature 68-72°F.

Ay one time the Bitterling was used in testing humans for pregnancy. This test does not give accurate results and has fallen into disuse.

The Bitterling which is an introduced species is local and rare. It is likely that the known colonies in Shropshire, Cheshire and Lancashire are the result of escapes from aquaria. It was discovered living wild in the first quarter of this century and is not yet known to be established in any other area in the British Isles: indeed it appears to be decreasing in numbers.

Habitat

The Bitterling is found in small, sandy lakes and to a lesser extent in canals and slow flowing rivers where mussels of the family Unionidae occur.

Spawning

From May to July the males develop an intense purple colour on the body, white tubercles on the head and red dorsal and anal fins, while the females develop a long ovipositor. By means of this the yellowish eggs which measure 3mm in diameter are laud in the mantle cavities of the Swan mussel, Anodonta cygnea, and other bivalves and the shed sperm from the make is taken in by the mussel and the larvae emerge through the inhalant syphon three or four weeks later when they have consumed their yolk sacs and measure 2cm (sic) in length.

Size

The Bitterling is a small fish which averages between 5 and 8cm in length and which is rarely longer than 10cm.

Diet

Small invertebrates make up most of the Bitterling's diet but a little plant matter is also eaten.

Identification

The Bitterling is a deep bodied fish which has a bluish grey back, paler blue or silvery sides and a white belly. The lateral line is restricted to between 4 and 7 scales near the head and there are 34 to 38 lateral scales. There is a bright metallic blue horizontal stripe on the caudal peduncle and the fins are pale grey often tinted with orange.

There are no similar species and no hybrids involving the Bitterling have been recorded from British waters.